As is known to those skilled in the art, optical fiber based sensing technology has been rapidly developed and widely used recently in biological and biomedical studies. Many of these studies employ conventional one-photon fluorescence (OPF) measurement techniques. However, there are a number of well-known advantages in using a multiphoton fluorescence including two-photon fluorescence (TPF) measurement technique. The small nonlinear excitation volume in the close proximity of the fiber tip enables local detection at a specific site. The use of near infrared light allows minimization of photodamage to living cells and drugs, in contrast to excitation by energetic UV photons. The large separation in wavelength between two-photon excitation and fluorescence emission facilitates elimination of detection of background noise. Finally, a single laser source may be used to excite a wide variety of fluorophores. Two-photon excitation arises due to the simultaneous absorption of two incident photons by a molecule. This excitation causes a ground-state electron to transition to an excited state of the fluorophore. Because two photons are required for each transition, the probability of excitation is dependent on the square of the instantaneous incident radiation intensity. Thus, an ultra-short-pulsed laser beam is usually needed for efficient excitation.
The recent introduction of optical fibers and fiber-optical components into conventional imaging systems has provided additional advantages. For example, excitation laser beam can be delivered deep into a targeted biological sample through an optical fiber, which otherwise is subject to strong scattering and absorption by biological tissues. In addition, using optical fibers, bulk optics and laser sources may now be placed remotely from the sample to be tested.
However, the use of conventional optical fibers leads to a number of disadvantages due to their physical limitations. Generally, there is a tradeoff between optimal excitation and optimal collection when using a single-mode fiber versus a multi-mode fiber. That is, single-mode fibers create higher laser peak intensity at the exit tip of the optical fiber when compared to multi-mode fibers. This higher laser peak intensity increases the nonlinear optical excitation rate. However, the lower numerical aperture of single-mode fibers suggests that multi-mode fibers have superior collection efficiency of optical signals such as fluorescence.
Accordingly, there exists a need in the relevant art to provide an optical fiber for use with the multiphoton fluorescence measurement technique that is capable of providing high laser peak intensity at the exit tip without compromising the fluorescence collection efficiency. Additionally, there exists a need to provide a dual-core optical fiber for use with two-photon fluorescence measurements that is capable of overcoming the disadvantages of the prior art.